Contents

A landing party from the USS Enterprise comprised of CaptainKirk, Scott, Dr.McCoy, and Uhura are on the Halkan homeworld, attempting to gain rights on behalf of the Federation to mine dilithium on their planet. The Halkan leader, a man named Tharn, tells Kirk that while they find him to be believable, it does not change their overall position and that the Halkan Council cannot permit the Federation to mine dilithium on their planet. Kirk says that they have shown them historical proof that the Federation's missions are peaceful. Tharn tells him they believe that the Federation is currently peaceful, but there's no guarantee that that will always be so. Tharn knows that the dilithium crystals represent incredible power, and even one life lost as a result of the power of their dilithium crystals would violate the Halkan's history of total peace, and the Halkans are all willing to die to prevent that. The Halkans are complete pacifists. Kirk says that while he admires the Halkan's ethics, he hopes to be able to prove theirs.

While these discussions are going on, an ion storm is passing through the Halkan system, causing strong thunder and lightning bolts. Kirk calls the Enterprise through his communicator and asks Spock for a report. Spock tells Kirk that it is a standard ion storm but rather violent and unpredictable. Kirk tells Spock to prepare to beam up the landing party and also to increase Enterprise's orbit to clear the disturbance of the storm. Kirk then asks Tharn when they may resume negotiations. Tharn tells Kirk the Council will meditate on the Federation's offer more, but that he shouldn't be very optimistic of any change. Just before the landing party can depart Tharn points out that with the weapons on board Enterprise, Kirk would have the might to force the crystals from the Halkans. Kirk tells him to consider that, while they do have such power, there is absolutely no chance that they would use it. He then calls for beam up and the landing party dematerializes.

Unfortunately, interference from the ion storm gives the transporter operator, Kyle, trouble as Spock enters the transporter room. Spock helps Kyle try to beam the landing party aboard, but they eventually disappear out of the transporter platform. Finally, the landing party does materialize and they start to step off the transporter pads, when Kirk looks up and sees Spock with a beard, the crew wearing much more flamboyant uniforms and offering Kirk a Roman-like salute, fist striking chest and then arm extending outward toward Kirk. Kirk and company are briefly shocked at the change, but manage to cover their shock. The changed Spock steps up to Kirk asking him for a status report on the mission. Not sure what to make of the situation, Kirk simply tells Spock that there was no change in negotiations. Spock asks Kirk if standard procedure should be initiated to which Kirk simply nods "yes," not knowing exactly what that means, but playing along. Spock steps over to the transporter console and contacts the bridge, telling Sulu to program a phaser barrage on the Halkan cities. Spock then asks if the Halkans have any military capabilities and Kirk tells him they don't. Spock says that it is regrettable that the Halkans have chosen suicide.

Spock then turns to Kyle and lectures him that he was supposed to compensate during the ion storm and that carelessness with the equipment cannot be tolerated. Spock then demands for Kyle's agonizer. Kyle begs Spock not to make him use it, saying he really tried to do what he was supposed to. One of the security guards reaches over and takes Kyle's agonizer and gives it to Spock, who places it on Kyle's shoulder causing him extreme pain for a few moments. Spock finally stops and lets Kyle slump down to the floor. Kirk and the others do their best to contain their shock and horror at what is happening.

"Captain's log stardate... unknown. During an ion storm the landing party has beamed back to the Enterprise and found it and the personnel aboard changed. The ship is subtly altered physically. Behavior and discipline has become brutal, savage."

Having finished with Kyle, Spock drops the agonizer and steps over to tell Scott that the storm caused some minor damage and there were some injuries requiring McCoy's attention. He then shows a bit of impatience when neither man moves to leave the transporter room. By this point, Kyle has recovered enough to tell Spock that the power beam jumped just as the landing party was about to materialize. Spock asks if it was due to Kyle's error, but Kyle tells him it was before his error. Spock then asks Kirk if they've suffered any ill effects, to which Kirk says that he does and tells McCoy to give them an examination. As the landing party stop to leave the transporter room, Kirk turns back to Spock and tells him to have the transporter circuits examined. Once having exited into the corridor, Scotty and Uhura begin to loudly ask Kirk what's happened, but Kirk silences them and tells them to wait until they are able to speak privately.

They walk through the hallway, which seems to have security officers at every intersection. Every crew member that Kirk encounters gives him the same Roman/Nazi-esque salute, which Kirk mimics. Every door on the ship seems to have a picture of the Earth bisected by a sword on it. Once they enter sickbay, McCoy goes over to a table and tells Kirk that everything is out of place, all messed up, and changed around, except for a spot where he spilled acid a year previous. Kirk asks everyone if any of them felt dizzy in the transporter beam; they all say that they did. Kirk says it happened twice. First they were in their own transporter chamber, then they faded, and upon finally materializing, they had appeared wherever they were. Scott says that the transporter lock could have been affected by the ion storm and they just materialized somewhere else. Kirk then realizes what has happened. They've somehow entered a parallel universe, where everything's duplicated... or, almost duplicated. Everyone contemplates the thought that they all likely have counterparts in the universe that they exchanged places with during transport, that there were similar storms on both universes disrupted both sets of transporter circuits. Kirk and crew are on this strange ship and their counterparts are on their Enterprise. Kirk realizes they have to use the computer to try to figure out how to get home. McCoy tells Kirk they can't let the Halkans be destroyed. Kirk tells Scott to go down to engineering and short out the main phaser controls, but to make it look like the storm blew out the standby circuits. Then he wants Scott to study the ship's technology as they're going to have to use it to get back home. Kirk also tells everyone to scramble the frequency on their communicators and to use them for private messages as the intercoms are most likely monitored.

Kirk then tells Uhura to go to her post and find out his exact orders from StarfleetCommand and if he has any options. Uhura stops and starts to tell Kirk she's scared, but Kirk reassures her that she's the only one who can do it and that he'll be right there. Reassured, Uhura leaves for the bridge. Meanwhile, Kirk and McCoy take a quick look at the ship's library.

Meanwhile, on the bridge, Chekov and Sulu are preparing coordinates for planetary targets when Uhura enters the bridge and assumes her station. Sulu leers over at her and steps up to her post, taking her face in his hand and telling her he could change her lack of interest in him. When Uhura tells Sulu he's away from his post, Sulu asks if either Kirk or Spock are there, and prepares to move in on her when she slaps his hand from her face hard. Just before he can move against her, Kirk enters the bridge from the turbolift. The entire bridge crew stands at attention and salutes Kirk, who returns the salute. Sulu quickly resumes his post, and Kirk steps over to Uhura, who tells him there was no storm damage and all stations are normal. She then quietly tells Kirk he is under orders to destroy the Halkans unless they comply and he has no alternatives. Kirk steps down and sits in the captain's chair, which is the same as his except for a raised back to it making the chair almost throne-like. When Sulu asks permission to fire, Kirk tells him to stand by. Meanwhile, in the phaser control, Scott is stopped by a security guard who asks Scott if he has authorization for this area; Scotty tells him he has the captain's orders, but the guard says he'll have to check with Security Chief Sulu. Scott says he'll do it, then after the door closes, he calls Kirk on the intercom and tells Kirk there's no damage to the phasers.

Spock enters the bridge and sees that the planet rotation is carrying the primary target out of phaser range. When Kirk won't let Sulu alter Enterprise's orbit, Spock tells him to lock onto the secondary city. Kirk tells Uhura to contact the Halkan Council so he can speak to them again. He then looks at Spock and tells him they have other things of value besides the dilithium crystals. Spock points out though that they have clearly demonstrated they will not cooperate by refusing the Empire. Spock tells him that standard procedure demonstrates that they make an example of them. When Sulu reports the secondary target rotating out of phaser lock, Kirk orders Sulu to put phasers on standby, which Spock notes is a serious breach of orders. Kirk tells Spock he'll make the reasons clear to him in his own good time.

At that point, Uhura tells Kirk she has the Halkan Council on channel. Kirk tells the leader it is useless to resist them, but the Halkans insist they are not resisting. Kirk gives them twelve hours to reconsider. However, Tharn tells Kirk that, be it 12 years or 12,000, they are ethically compelled to deny the Empire's demand for their dilithium crystals, for the Empire would use their power to destroy. Kirk tells them destruction is the Enterprise leveling their planet and taking what they want, and that the Halkans will die as a race. Tharn says they will willingly do so to preserve their culture. Kirk closes the channel and tells Sulu to turn phasers off. Spock says a twelve-hour reprieve is unprecedented. Kirk leaves for his quarters, telling Uhura to have McCoy and Scotty meet him there. On his navigation panel, Chekov presses two buttons and leaves with Kirk. Spock tells Kirk before he enters the turbolift that he must report Kirk's conduct to Starfleet Command. Kirk tells Spock he is at liberty to do so.

In the turbolift, Chekov asks Kirk if he wants to go to Deck 5; Kirk nods that he does. Just as the doors open and Kirk prepares to walk out, he is hit forcefully in the mouth by a crewman, and held against the wall by two others. Chekov pulls out a phaser and tells Kirk that with his death, the entire crew will advance in rank and that no one would dare question the assassination of a captain who disobeys the prime orders of the Empire.

"Captain's log, supplemental. I command an Enterprise where officers apparently employ private henchmen among the crew, where assassination of superiors is a common means of advancing in rank."

Just as Chekov is about to shoot Kirk, Wilson knocks the phaser out of Chekov's hand, and throws him into the corner, knocking him out. He then gets one of the crewmen off of Kirk and shoots him dead, vaporizing him with his own phaser. Kirk wiggles away from the other guard and knocks him across the deck and Wilson shoots him dead with his phaser. Just at that moment, Kirk's henchmen appears out of the turbolift; Wilson tells one of them that he helped Kirk, which Kirk confirms. He then turns his attention to Wilson, who tells Kirk that Chekov was going to make him a chief and asks if Kirk might make him an officer. Kirk says that Wilson is, from then on, working for him. Wilson asks if he's commissioned, and Kirk smiles at him and tells him that he's now in line, he might even make captain. Wilson smiles; after an enthusiastic "Yessir," Kirk knocks Wilson out and says he won't be captain on Kirk's ship. Farrell asks Kirk if Chekov should go to "the booth"; Kirk assents.

Arriving at Kirk's quarters, he tells McCoy and Scott to watch their backs, as the officers move up by assassination. Scott tells Kirk that Sulu is security chief and compares him to the ancient Gestapo. McCoy compares sickbay to a chamber of horrors where two of his assistants were betting on how long it would take an injured man to pass out from the pain. Scott then tells Kirk the technology is virtually identical, with only variations in instrumentation. When Kirk asks about star readings, Scott tells him everything is exactly as it should be, except for them.

Kirk then decides to try the library computer. At first Kirk is surprised to hear the computer have a male voice. He then begins a program of classified security research, accessible to only him and Scotty. Using all the data from the ion storm, Kirk has the computer work on the idea to see if a sufficiently strong enough ion storm could cause a power surge in transporter circuits which create a momentary interdimensional contact with a parallel universe. The computer answers "affirmative." Kirk then asks if people, in the act of beaming through such a storm, could they transpose accidentally with their counterparts in the other universe, to which the computer answers "affirmative." Kirk then asks if such conditions could be artificially-created using the ship's power, to which the computer, again, answers "affirmative." Kirk records the research onto a record tape and gives it to Scott. Scott tells Kirk that he'll need help, but Kirk would be too conspicuous. At that point, he drafts McCoy to help him. "I'm a doctor, not an engineer," McCoy states. "Now, you're an engineer," Scott replies.

As Scott steps off to another terminal in Kirk's quarters to do some research of his own, McCoy asks Kirk what kind of people they are in this universe. So Kirk asks the computer for a readout of the current commander. The computer tells him that Captain James T. Kirk succeeded to command the ISS Enterprise through assassination of Captain Christopher Pike. Kirk's first two actions were suppressing a rebellion by destroying the rebels' home planet and executing 5,000 colonists on Vega IX.

Kirk quickly cuts off the record, having heard more than enough. Scott tells Kirk that their plan can work by using the automatic transporter setting. Unfortunately however, their interruption of the engine circuits will appear on Sulu's security board. Kirk says he'll get Uhura to create a diversion to distract Sulu's attention as soon as he's ready. Kirk tells everyone to get back to their posts and keep him appraised. After Scott leaves, McCoy and Kirk ponder what their counterparts are doing back in their universe and on their Enterprise.

At that moment, on the prime universeEnterprise, the counterpart Kirk is being forcefully dragged to the brig by two security officers. Kirk demands that the security release him, but they ignore him. Spock Prime is waiting for them at the brig. Kirk tries to get Spock to let him go. The security officers throw Kirk into the brig with the other counterparts who angrily ask what's going on and they also clearly yell at Spock. Kirk threatens to have Spock and the security guards executed. Spock tells him that's not likely as his authority is extremely limited in that regard. Spock tells Kirk that the four of them will remain in the brig and in custody until he can figure out how to get them back to wherever it is they belong. Kirk says the whole galaxy's gone crazy, wonders about his uniform, about Spock's beard, and what has happened to his personal guards. Spock says he cannot answer those questions at this time. Kirk then tries to bribe Spock with either credits or perhaps his own command. Spock theorizes that some kind of interesting transference has taken place. Kirk asks Spock what might buy him, perhaps power? Spock can only reply with "fascinating." Kirk yells at Spock as he's walking away, telling Spock that he can get power for him.

Kirk Prime and Mirror Spock walk

Meanwhile, back on the mirror Enterprise, Kirk Prime is walking through a corridor when he's met by Spock, who has a guard of his own – another Vulcan. Spock tells Kirk that he's pleased Kirk foiled Chekov's plan, stating that he does not want to be captain, he prefers his scientific duties, and that he is, quite frankly, content to be a lesser target. Kirk tells him that he is logical as always. At that moment, a switch is thrown and we hear Chekov screaming. Kirk turns and sees Chekov in some kind of booth, surrounded by the guards that took him away. Spock tells Kirk the agony booth is a most effective means of discipline, and then asks Kirk if he's ordered full duration. Kirk says he hasn't decided – a decision which surprises Spock, as Chekov's act warrants death. Kirk angrily repeats that he hasn't decided.

Spock submits that that is Kirk's business, then asks if he intends to continue in this unusual course of action regarding the Halkans, as they are in conflict with standard Imperial procedure and that Kirk cannot ignore the consequences. Kirk asks Spock if that is a threat; Spock tells Kirk that he is merely stating facts. Spock says that he has found Kirk to be an excellent officer, and that their missions together have been both successful and profitable. However, Spock will not permit Kirk's aberrations to jeopardize his position. Kirk asks Spock if he thinks they should destroy the Halkans; Spock tells him that "terror must be maintained, or the Empire is doomed. It is the logic of history."

Kirk wonders if they've perhaps gone too far with their conquest, stating that that part is easy, but controlling the masses are not. Spock says he does not want to have to find himself opposing Kirk, but if Kirk persists in his confusing, inexplicable behavior, he will have no choice. Kirk tells Spock that it is his concern and not Spock's. Kirk tells Spock that he would find Kirk to be a formidable enemy; Spock says he knows that, and asks if Kirk is aware of the reverse. After Spock leaves, Kirk tells Farrell to release Chekov from the booth and confine him to quarters.

Meanwhile, outside engineering, Scott distracts a guard, McCoy is able to hypo him unconscious. They drag him inside the door and climb up the ladder to the emergency manual monitor and begin to do some of their work.

Kirk goes to his quarters and finds a woman asleep on his bed. She tells him they had a difficult time in the chemistry lab cleaning up the mess after the storm, but nothing compared to what he's gone through, referring to his earlier incident with Chekov. Kirk says that Chekov gambled and Kirk won. Marlena says that Kirk got lucky, and states she's surprised as to how easily he was caught off-guard. Kirk tells her he was preoccupied. She asks if he's still in trouble with Starfleet Command. She can't figure out what kind of scheme he's got in mind. She figures that either the Halkans have something he wants, or he's trying to scheme a way to advance to the Admiralty or perhaps the Cabinet. Kirk says if he's successful, he may go further than that. Moreau asks if she can know what his plan is, thinking it only fair if she's to be the woman of a Cæsar.

After a brief kiss, the intercom chimes and Kirk steps over to it. Spock is calling him, telling him that he's received a private message from Starfleet Command and that by telling Kirk about it, he is violating regulations. Spock tells Kirk that he must wait until planet dawn over principal target to permit Kirk to carry out their mission; if Kirk does not discharge his duties, then Spock is ordered to kill him and proceed against the Halkans... as the new captain of the Enterprise.

"Captain's log, stardate unknown. We are trapped in a savage parallel universe from which we must escape within four hours or I will face a death sentence at Mr. Spock's hands."

Moreau and Kirk drink a toast to Spock as the only man aboard with the decency to warn Kirk, and he'll die for it. Kirk says he does not intend to kill him. Moreau asks if he's going to act against the Halkans before the deadline and Kirk says he won't, but he'll also avoid killing Spock. Moreau asks if Kirk is just going to get Spock and his men out of the way; Kirk says he'll get out of Spock's way. Moreau asks if she should activate the Tantalus field so Kirk can monitor him; not knowing what she means, Kirk tells her to go ahead, and watches as she touches a couple of areas on what appears to be a decoration on the wall, revealing a monitor screen and some buttons. Moreau tells Kirk she hates the device. Playing for time while she unwittingly shows him what the device is and how it works, Kirk says "it's not so bad". Moreau replies that of course he'd think that, as it is what made him captain of the Enterprise. She then wonders how many people Kirk has simply wiped out of existence by the touch of a button (thoughtfully pointing to the button as she speaks) and then contemplates the humor in how "the great, powerful Captain Kirk, who owes everything to some unknown alien scientist and a plundered laboratory." Kirk and Moreau mutually agree then that if one does not take advantage of one's opportunities, one does not rise to command a starship or even higher.

Observing Spock, Moreau offers to go ahead and kill him, but Kirk stops her and deactivates the device. Moreau says that if Spock fails to kill Kirk, then he'll still be executed. Kirk says that he'll make certain the circumstance of Spock's failure will clear him. Moreau is amazed that Kirk isn't even afraid of Starfleet Command. She wonders just how much power is Kirk expecting to accumulate and then wonders how she'll fit into the circumstances. Kirk asks Moreau how she wants to fit in, to which she smiles and then walks through a door, ostensibly to her end of the cabin, leaving Kirk alone.

Alone, Kirk calls Scott on his communicator and tells him they have to get out of there inside of three hours as Spock has orders to kill him unless he completes their military mission. Scott tells him that the two-way transmission affected local field densities within the two universes and they've got to escape within a half-hour or they'll be stuck in the mirror universe for the rest of their lives. Scott needs Kirk to get to the transporter room in about ten minutes and free up the board so Scott can lock in. Kirk says after he leaves the transporter room he'll meet Scott and McCoy in sickbay afterward.

In his quarters, Spock has been curious about the computer activity and inquires the computer about it. The computer tells him that the captain and Scott are conducting a classified security research under Kirk's voice index log. Spock turns to contemplate that when he notices a beep on his computer and after he activates it, asks Sulu why he is monitoring Spock's communications. Sulu tells Spock that his security board had detected extensive use of computers and he was going to inform Spock. Sulu says that he can figure out what Starfleet told Spock and tells him that he suspects that Kirk is working on escape or defense. When Spock tells Sulu it's his business, Sulu tells Spock he hopes Spock succeeds, because the order would fall on Sulu next, and Sulu is concerned about how Kirk's enemies have the unfortunate habit of disappearing. Spock tells Sulu that if he succeeds, Sulu will find himself a step closer to the captaincy. Spock says he does not want to command the Enterprise, but if it should happen, Spock reminds Sulu that his operatives would avenge his death, and some of them are Vulcans. That statement seems to frighten Sulu a bit, and he closes the channel.

Back in Kirk's quarters, Moreau reappears, wearing a much more seductive outfit. She admits to being a bit out of practice and laments that it's difficult for her to appear perfectly as a woman constantly. Kirk says that he's never seen any woman come closer to perfection. She says she remembers how he used to talk that way. Kirk says he still does. When she asks for proof, Kirk says he has to go. Moreau is angry about being put off for one task or another. She then realizes that their relationship must be over. Moreau tells Kirk that Commander Kenner will take her for a time. When she prepares to call a yeoman for assistance to move out, Kirk tells her that's unnecessary. Moreau wonders if Kirk is feeling sorry for her. Then she demands a transfer, saying on the Enterprise she's humiliated but on another ship she can hunt for another man. She then says she's got her rank, that she's been a captain's woman and she likes it, and if it requires she go through every officer in the fleet, she'll be one again. Kirk says she could, and when Moreau turns to strike him, Kirk stops her and tells her he simply meant that Moreau could be anything she wanted to be. He then kisses her very passionately. Moreau says it's been a long time since he kissed her like that and based on Kirk's mercy toward Spock and toward her, calls him a stranger. She then asks if she's his woman. Kirk tells her she's the captain's woman until he says she's not. He then leaves their quarters. Moreau turns on the Tantalus field and watches Kirk go into the turbolift.

In the turbolift, Kirk calls Uhura and tells her that Scott should be calling her soon. He asks if she's prepared; she says she is, and that she will do her best to keep Sulu distracted. Kirk wishes her good luck and then closes the channel.

Uhura distracts Sulu

At this point, Scott and McCoy have climbed up into a Jefferies tube and, just before Scott begins to make the alterations, he signals Uhura. Uhura walks down to Sulu and chides him for not being very persistent, reminding him that he's ignoring the rules of the game. She's supposed to protest and then he's supposed to come back. Sulu puts his arms around her and tells her she's making sense now. Uhura says she was getting bored, but of course this isn't the time. Sulu, meanwhile, is kissing Uhura's neck and tells her any time's a good time. While he is kissing her, the beep on his console from Scott's work is detected, but goes unnoticed. When the beep stops, she smiles, backs away and then backhands Sulu back into his seat, telling him she's changed her mind again. An angry Sulu tells her she takes a lot of chances, but Uhura draws her knife and tells Sulu he does, too. She keep Sulu at bay with her knife and has one of the relief officers take over for her and she leaves the bridge. While in the turbolift, she contacts Scott and tells him that it's all clear and she's headed for sickbay.

In the transporter room, Kirk is working on unlocking the controls on the transporter console when Spock steps in and detains him. After confiscating Kirk's weapon, Spock asks Kirk what he's doing, but Kirk will give no information and tells Spock to go ahead and shoot him. Spock says that Kirk is too inflexible and disciplined once he's made up his mind. Conversely, Dr. McCoy is sentimental and soft, so he'll be able to get the information he needs from him. Kirk threatens Spock, but Spock reminds Kirk that he's the one with the phaser and he does not intend to simply disappear as so many of Kirk's opponents have previously. Spock then leads Kirk to sickbay, where he finds the rest of the landing party waiting on Kirk. Kirk turns and attacks Spock and the others join in. Spock is able to handle all four rather easily, although Kirk does put up more of a fight than the others. Finally, Uhura hands Kirk a ceramic which he smashes over Spock's head, rendering him unconscious. Scott tells Kirk they have maybe 15 minutes to go and McCoy demands they help him get Spock on the examination table as Spock will die without immediate treatment.

After complying, Kirk asks Scott if everything's ready for them to go; Scott says it is and tries to get McCoy to hurry, as they are risking not going home. McCoy says they have time and tells Scott to shut up. McCoy asks Kirk if he should stop, but that his work will only take a minute. Kirk mentions that this Spock is very much like their own Spock, and that McCoy can continue. Just then, Sulu walks in with three security guards. When Kirk asks what he is doing, Sulu says that Spock is going to succeed in his order to kill Kirk... apparently. Kirk will also appear to have killed Spock after a fierce battle. Sulu calls it regrettable, but it will leave him as captain.

As Kirk and company prepare to defend themselves against Sulu, one of Sulu's henchmen suddenly disappears. Back in Kirk's quarters, Marlena has been monitoring Kirk ever since he left their quarters and she has learned who he really is. She vaporizes the other guards, leaving Sulu alone to fight Kirk, and Kirk knocks out Sulu fairly easily. Scott tells Kirk they only have ten minutes. Kirk tells McCoy that his time is up, but McCoy can't let Spock die and asks for only five minutes; he guarantees he'll be there. After Kirk and the others leave, McCoy gives Spock a hypo which wakes him up. Spock then grabs McCoy's arm and asks why Kirk has let him live. McCoy won't answer him, so Spock backs McCoy up against a wall and then mind melds with him.

In the transporter room, Marlena is waiting on the landing party. Kirk sends Scott to activate the transporter and expresses gratitude to Marlena for saving them. Marlena asks Kirk to take her with him. Kirk says he can't, that their power is balanced for only four and if they try it with five, they could all die. Marlena points out there are only three of them. Kirk says one is coming and that he wishes he could help her. Marlena points a phaser at Kirk then, and Kirk tells her if she kills them, she'll still stay. Uhura approaches her from behind and quickly disarms Marlena. Kirk asks for the time and Scott tells them they have five minutes when suddenly the power cuts. Kirk asks if auxiliary power will work and Scott says getting the power is no problem but the automatic setting is linked to the transporter main and if they bypass, someone will have to stay behind to operate the controls manually.

Scott volunteers to stay, but Kirk tells him and Uhura to get to the transporter chamber. Scott begins to protest, even calling Kirk "Jim," but Kirk makes it an order and Scott reluctantly complies. Kirk goes around to the transporter controls, and then Spock enters with McCoy. Spock has also learned the landing party's true identities through his mind meld and it was him that had the transporter power cut so they couldn't beam out before he arrived. Spock gives McCoy to Scott, and then orders engineering to reactivate the main transporter circuits. Kirk calls Spock "a man of integrity in both universes." Spock says that they must get back to their universe and Spock must have his captain back and that he will operate the transporter.

In the two minutes and thirteen seconds that remain, Kirk asks how long before the Halkan's prediction of galactic revolt is realized. Spock, as ruthlessly logical as his counterpart, has calculated that it will occur in approximately 240 years, and the inevitable outcome will be the overthrow of the Empire. Kirk then challenges the "illogic" of Spock continuing to serve an Empire that he knows is certain to collapse. Kirk then asks if change is inevitable, and for the good, wouldn't logic insist that he be part of it? Spock tells Kirk one man cannot summon the future, but Kirk tells him one man can change the present.

"Be the captain of this Enterprise, Mr. Spock! Find a logical reason for sparing the Halkans and make it stick, push till it gives! You can defend yourself better than any man in the fleet."

Spock tells Kirk that a man must also have the power to effect change. With a glance toward Moreau, Kirk reveals the existence of the Tantalus field, with which Spock will be "invincible." Spock appears intrigued. Kirk asks him to make the choice: the past or the future, tyranny or freedom. Kirk then steps into the transporter chamber and then tells Spock "in every revolution, there's one man with a vision." Spock says only, "Captain Kirk, I shall consider it," and then energizes the transporter.

Reappearing in the their own universe, the landing party steps off the transporter pad and are happy to see a clean shaven Spock, who welcomes them home.

On the bridge, as the Enterprise leaves orbit of the Halkan homeworld, Kirk asks how Spock detected their counterparts so quickly. Spock tells Kirk that as civilized men, they were able to impersonate barbarians much easier than it was for barbarians to imitate civilized men. Spock speculates that the counterparts returned to their Enterprise at the same time the landing party arrived home. Kirk speculates that his counterpart might be in for a few changes, though. McCoy says he thinks he liked Spock with a beard better, as it gave him character. Kirk mentions how easily the counterpart Spock fit into the other universe, and speculated that Spock might be a bit of a pirate at heart. Spock comments on how he observed their counterparts also, calling them "...brutal, savage, unprincipled, uncivilized, treacherous; in every way splendid examples of Homo sapiens, the very flower of Humanity." Spock also adds that he found them quite refreshing. Kirk says he's not sure, but he thinks they've been insulted. McCoy, however, is sure.

At that moment, Lt. Marlena Moreau walks on the bridge with a report on a PADD for Kirk. McCoy, Scott, and Uhura are shocked to see her, and when she arrives at Kirk's chair, he asks Moreau her name and she tells him she was just assigned to the Enterprise last week. After she walks away, Spock asks Kirk if he knows her, citing his reaction as one of recognition. Kirk tells Spock that he hadn't met her before... exactly. He hopes that he and Moreau can become friends.

"Captain's log, stardate unknown. During an ion storm, my landing party has beamed back to the Enterprise and found it, and the personnel aboard, changed. The ship is subtly altered, physically. Behavior and discipline has become brutal, savage."

"Captain's log, supplemental. I command an Enterprise where officers apparently employ private henchmen among the crew, where assassination of superiors is a common means of advancing in rank."

"Captain's log, stardate unknown. We are trapped in a savage parallel universe from which we must escape within four hours, or I will face a death sentence at Mr. Spock's hands."

Jerome Bixby based this episode very loosely on his own short story "One Way Street". In the original draft script, Kirk traveled to the parallel universe alone and the parallel universe Federation was battling a race called the Tharn. This name was later given to the leader of the Halkan Council, although it is not spoken on screen. [1]

During the filming of this episode, Barbara Luna had strep throat and a 103-degree fever, so her kissing scenes with Shatner were postponed for three weeks. (TV Guide: Vol. 44, No. 34, Issue #2265, p. 28)

As mirror Sulu is the security chief as well as the helmsman, George Takei wears a red uniform in this episode – since he normally wore gold, and had worn science blue as an astrophysicist in "Where No Man Has Gone Before", this makes Takei the first Trek actor to wear all three uniform colors.

This is also the only episode in which Uhura is seen in a moving turbolift.

This is one of two episodes where Spock is knocked unconscious in a fight (the other being "Friday's Child" where a Capellan knocks him out with a sword after Kirk prevents Eleen's death) contradicting the book Star Trek Lives, in which Leonard Nimoy states that Spock was never knocked out in a fight (although it must be noted, that it's Spock's mirror universe counterpart who is knocked unconscious in this episode).

In Act Four, Kirk asks Mirror Spock how long it will be before the Halkan prediction of galactic revolt is realized. Within the context of the aired episode, this is an unlikely question for Kirk to pose because there is no prior mention or discussion of such a prediction. However, James Blish, the author who adapted most of the TOS scripts into stories in the late 1960s and early 1970s, and who often worked from early drafts of final shooting scripts, provides a clue to this "missing discussion" in his "Mirror, Mirror" adaptation in Star Trek 3. After Tharn tells Kirk "To preserve what we are," he continues, "Yes. Perhaps someday your slave planets will all defy you, as we have done. When that comes, how will your starships be able to control a whole galaxy?"

This is the second and final appearance of the eleven-foot model Enterprise, which orbits from right-to-left in the teaser. When the transposition to the parallel universe occurs, the ISS version is orbiting in the opposite direction. Although the episode was produced in 1967, the footage of the ship was filmed for "Where No Man Has Gone Before" in 1965, indicated by the heightened bridge dome, the oversized deflector dish, and the unlit Bussard collectors (complete with spires). However, in "Where No Man Has Gone Before", there were no rounded "nodes" at the rear of the nacelles, which appear in this episode. The first time the ship orbited in this direction, in TOS: "Shore Leave", scenes of the revamped production model were simply reversed as can be seen by the backward registration. In this episode, though, the model was shot with reversed nomenclature as the port side was unfinished. It is unknown why so little of this footage was used during the series. Despite the effort expended to highlight the opposite nature of the mirror universe, the Enterprise is seen orbiting from left-to-right in all of the scenes after the main title.

A modified brig makes its debut here. Its location on the set was in the short hallway leading to the Engineering set. The brig seen in "Charlie X" was in the main hallway, but featured many of the same set pieces. However, the force field projectors did not move when someone attempted to get out of the new version.

To further denote the inverted nature of the parallel universe, phasers are worn upside-down on the left hip.

After McCoy tranquilizes the man guarding the door to engineering, the arm of the stage hand operating the door can be seen as the door opens and Scott and McCoy pull the man into engineering.

This episode is often spoofed or parodied in various other television programs. The aspect of evil counterparts from a parallel universe and the presence of goatees on such characters are particularly popular. See Star Trek parodies and pop culture references for such pop culture references.

One such reference or tribute is the noted progressive rock band called "Spock's Beard."

In the late 1980s, the pop band Information Society sampled Kirk's line, "It is useless to resist us," at the very beginning of their song "Walking Away", as well as "In every revolution, there's one man with a vision," on "Over the Sea".

This episode is parodied/referenced in the Mystery Science Theater 3000 episode featuring "Last of the Wild Horses." During the host segments of this episode, the evil Dr Clayton Forrester and TV's Frank propose sending a "matter transference device" to the Satellite of Love during an ion storm, and for the rest of the episode, Mike Nelson and Crow T. Robot are seen as their evil counterparts on Earth, sending bad movies to the Satellite of Love to torture Forrester and Frank – the reverse of the usual situation. Mike and Crow play roles similar to Mirror Kirk and Spock (respectively), including wearing costumes similar to those worn by Mirror Kirk and Spock. Prime Universe's Tom Servo and Gypsy try and figure out how to get back to their own universe.

"Fairest of Them All", the third episode of fan film series Star Trek Continues, starts with the original (re-acted) ending of this episode and continues with the story in the parallel universe.

TV Guide ranked this as the third best Star Trek episode for their celebration of the franchise's 30th anniversary. (TV Guide August 24, 1996 issue)

"Mirror, Mirror" was the tenth of the remastered episodes of The Original Series to air. It premiered in syndication on the weekend of 11 November2006. Among the changes made to the episode, the agonizer effect was enhanced both during Chekov's torture, and when Spock used it on Kyle (the agonizer sparking and leaving Kyle's shoulder with a slight glow). The Halkan planet was touched up, as was the ISS Enterprise, reflecting the Enterprise model from "The Cage", but now appropriately marked "I.S.S." and given a dark, gunmetal gray coloration, and now always shown orbiting from right-to-left, the reverse of its counterpart's movement in all other episodes.